The origins, specificity, and potential biological relevance of human anti-IgG hinge autoantibodies.

Human anti-IgG hinge (HAH) autoantibodies constitute a class of immunoglobulins that recognize cryptic epitopes in the hinge region of antibodies exposed after proteolytic cleavage, but do not bind to the intact IgG counterpart. Detailed molecular characterizations of HAH autoantibodies suggest that they are, in some cases, distinct from natural autoantibodies that arise independent of antigenic challenge. Multiple studies have attempted to define the specificity of HAH autoantibodies, which were originally detected as binding to fragments possessing C-terminal amino acid residues exposed in either the upper or lower hinge regions of IgGs. Numerous investigators have provided information on the isotype profiles of the HAH autoantibodies, as well as correlations among protease cleavage patterns and HAH autoantibody reactivity. Several biological functions have been attributed to HAH autoantibodies, ranging from house-cleaning functions to an immunosuppressive role to restoring function to cleaved IgGs. In this review, we discuss both the historic and current literature regarding HAH autoantibodies in terms of their origins, specificity, and proposed biological relevance.

Cleavage of IgGs by proteases associated with invasive diseases: an evasion tactic against host immunity?

The effective functioning of immunoglobulins and IgG mAbs in removing pathological cells requires that the antigen binding regions and the Fc (effector) domain act in concert. The hinge region that connects these domains itself presents motifs that engage Fc receptors on immune effector cells to achieve cell lysis. In addition, sequences in the lower hinge/CH2 and further down the CH2 region are involved in C1q binding and complement-mediated cell killing. Proteolytic enzymes of little relevance to human physiology were successfully used for decades to generate fragments of IgGs for reagent and therapeutic use. It was subsequently noted that tumor-related and microbial proteases also cleaved human IgG specifically in the hinge region. We have shown previously that the "nick" of just one of the lower hinge heavy chains of IgG unexpectedly prevented many effector functions without impacting antigen binding. Of interest, related single-cleaved IgG breakdown products were detected in breast carcinoma extracts. This suggested a pathway by which tumors might avoid host immune surveillance under a cloak of proteolytically-generated, dysfunctional antibodies that block competent IgG binding. The host immune system cannot be blind to this pathway since there exists a widespread, low-titer incidence of anti-hinge (cleavage-site) antibodies in the healthy population. The prevalence of anti-hinge reactivity may reflect an ongoing immune recognition of normal IgG catabolism. Tumor growth and bacterial infections potentially generate hostile proteolytic environments that may pose harsh challenges to host immunity. Recent findings involving physiologically-relevant proteases suggest that the potential loss of key effector functions of host IgGs may result from subtle and limited proteolytic cleavage of IgGs and that such events may facilitate the incursion of invasive cells in local proteolytic settings.

Genetic diversity at the hinge region of the unique immunoglobulin heavy gamma (IGHG) gene in leporids (Oryctolagus, Sylvilagus and Lepus).

Unlike other species, European rabbit (Oryctolagus cuniculus) possesses only one immunoglobulin gamma class. Allelic diversity at the Ig (immunoglobulin) gamma constant region encoded by the unique IGHG (immunoglobulin heavy gamma) gene is moreover much reduced. In the European rabbit, the genetic variation at IGGH hinge region is limited to a single nucleotide substitution, which causes a Met-Thr interchange at amino acid position 9 (IMGT hinge numbering). We have analysed the diversity at this region more in-depth by, (1) analysing the allelic variation in 11 breeds of domestic European rabbit (Oryctolagus cuniculus cuniculus), and (2) sequencing the gamma hinge exon in wild specimens of six species of rabbit (Oryctolagus and Sylvilagus) and hares (Lepus), including the two Oryctolagus subspecies (O. cuniculus cuniculus and O. cuniculus algirus). It appeared that among leporid species, amino acid changes occur exclusively at positions 8 and 9. However, while position 8 is occupied by either Pro or Ser residues, four different residues can occur at position 9 (Met, Thr, Pro and Leu). This variation concerns sites of potential O-glycosylation and/or proteolytic cleavage, suggesting that the underlying genetic diversity could be the outcome of selection. Preservation of the gamma hinge polymorphism in domestic stocks could therefore be important. We report here a polymerase chain reaction/restriction fragment length polymorphism protocol that has allowed the monitoring of the heterozygosity levels at the gamma hinge in 11 breeds of domestic European rabbit.